1. Technical Field of the Invention
The present invention relates generally to color image forming apparatus, and more particularly, to a color image forming apparatus wherein a plurality of developing units are arranged and supported on a movable support member in the direction of its reciprocating motion, and each one of the developing units are selectively used by changeably moving them to the position of developing process with reciprocating motion of the movable support member.
2. Brief Description of the Related Art
Such a kind of developing device is disclosed in Japanese Laid-Open Patent Application No. 57-204567. In the device, it is arranged that one of the developing units which does not interfere with a photoconductor during its movement in the course of reciprocating motion is selected and positioned for use at the developing position opposite to the photoconductor. In such a method of changeably positioning the developing units, there is arranged a safety space between the photoconductor and the developing unit which is positioned at the developing position on the course of reciprocating motion in order to prevent interference with each other.
Accordingly, when a developing unit is positioned at the developing position, it is pushed out toward the photoconductor and approaches to the photoconductor to be ready for developing process, while when the developing unit is changed to another one, the pushing action is released to return the developing unit to the course of the reciprocating motion so as not to interfere with the photoconductor.
In an image forming apparatus which is designed for forming a colored image, a balancer is used to balance with gross weight of a movable support member in order to reduce the driving torque which reciprocates the movable support member since three or four developing units are supported on the movable support member and making the gross weight so large.
Periodic maintenance operation is needed to clean the toner powder and its overflow, to remove clogged tone, and to change the developer according to the life of the carrier of a two-component developer. During the maintenance operation, all of the developing units are taken out. It is considered to be rational in this case to take out the developing units from the rear side of the movable support member in view of the supporting mechanism of the movable support member and the developing units, and the guide mechanism in the direction of movement toward and away from the photoconductor on the support member. However, at the rear side of the course through which the developing units reciprocate, there is arranged a pressing means to push out the developing unit positioned at the developing position toward the photoconductor. Accordingly, when the movable support member is moved to the position of any one of the developing units to be operated, the pressing means is positioned at the rear side of said developing unit.
Thus, when all the developing units are taken out, it has heretofore been performed in such a manner that the developing units which are not obstructed by the pressing means are first taken out, and then the developing unit positioned opposite to the pressing means is moved to another position so that it may also be taken out. Such an operation is troublesome and requires much labor.
Moreover, in the case of an apparatus wherein a balancer is activated against a movable support member, the movable support member and the balancer are out of balance by taking out even one developing unit, and the moving force by the balancer acts on the movable support member in accordance with the degree of unbalance. A lock means is, therefore, needed to lock the movable support member at a predetermined position. However, after some of the developing units have been taken out, the lock of the movable support member must be released in order to move the last developing unit to the position where it is removable. In such case, the developing unit is detrimentally forced to move with large force since the balance between the movable support member and balancer was broken by taking out developing units, and making the operation difficult and dangerous.
Conventionally, the movable support member is arranged to be locked only at the number of positions corresponding to the number of developing units, and therefore, when the developing unit on the lowest stage is moved to the developing position, it may happen that the movable support member is not locked completely. In such a case, if one of the developing units is taken out without knowing that, the balance between the movable support member and the balancer will be broken, and the movable support member is moved upward, which eventually causes damage to the main body of image forming apparatus and the like.
Further, the conventional movable support member is arranged to move each one of the developing units only to the developing position, and therefore, one of the developing units is stopped only at the position opposite to the photoconductor. The developing unit facing the photoconductor on the movable support member is released from pressure toward the photoconductor after developing process. However, the release function may be so incomplete that the developing unit and the photoconductor may be kept in close vicinity to each other. Under the circumstances, lumps of toner are stuck to the circumference of a developing roller, potentially causing the toner to scratch the surface of the photoconductor and the developing roller.